Just as you can have a closed off space in a vehicle (think limo) to drink in, I'm sure it will get upheld you can do the same with cannabis. But like a lot of these things, they gotta arrest someone who wants to fight the charges, to put anything in the books. Till then it's still a grey area.
Besides, I was pretty sure I could smoke pot in most cabs in Denver before the clown show...I always asked first, tipped well (unless they said no, then just a round up to the dollar). Most just said 'roll down the window', others did it for me.